1. Field of the Invention
The present invention relates to an optical pickup apparatus. In particular, the present invention relates to an optical pickup apparatus using a reduced number of parts.
2. Description of the Related Art
In these days, CDs (compact discs), DVDs (digital versatile discs) and the like are widely in use as optical disc media for information recording/reproduction. In order to record/reproduce information in an optical disc medium, a laser beam to be collected by an objective lens needs to be precisely focused on an information recording layer included in the optical disc medium. To this end, some controls are performed. Examples of the controls include: a focusing control for correcting an error which takes place when the laser beam is out of focus on the information recording layer in the optical disc medium; and a tracking control for correcting an error which takes place when a spot of the laser beam collected on the information recording layer deviates from a predetermined track center.
Well-known examples of the focusing control include a focusing control employing a differential astigmatism method. The focusing control employing the differential astigmatism method is performed as follows. First of all, the laser beam is diffracted by use of a diffraction grating or something similar. Thus, a zero-order beam, a plus first-order beam and a minus first-order beam are produced. Subsequently, the zero-order, plus first-order and minus first-order beams are cast on the information recording layer in the optical disc medium. Reflecting beams, which are the zero-order, plus first-order and minus first-order beams reflected off the information recording layer, are subjected to astigmatism by a cylindrical lens or something similar. The reflecting beams of the zero-order, plus first-order and minus first-order beams, which have been subjected to the astigmatism, are received by three quadrant photo-detectors provided in a photo-detecting device.
Referring to FIG. 7A, a first light receiving region 211, a second light receiving region 212, and a third light receiving region 213 are placed in a PDIC (photo detector integrated circuit) used for the differential astigmatism method. The zero-order, plus first-order, and minus first-order diffracted beams are received by these light receiving regions. In this respect, the first light receiving region 211 is formed from light receiving portions A′, B′, C′, D′; the second light receiving region 212 is formed from light receiving portions E′, F′, G′, H′; and the third light receiving region 213 is formed from light receiving portions I′, J′, K′, L′.
This drawing indicates shapes, which the laser beam cast on the light receiving regions takes on when the zero-order beam is focused on the information recording layer in the information recording medium, with circles drawn by chain lines. When the zero-order beam is focused on the information recording layer in the information recording medium, the light reception pattern of the reflecting zero-order beam is circular, and the reflecting zero-order beam is cast on the light receiving portions A′ to D′ equally. Similarly, the light reception pattern of the reflecting plus first-order beam is circular, and the reflecting plus first-order beam is cast on the light receiving portions I′ to L′ equally. Similarly, the light reception pattern of the reflecting minus first-order beam is circular, and the reflecting minus first-order beam is cast on the light receiving portions E′ to H′ equally.
In addition, this drawing indicates shapes, which the laser beam cast on the light receiving regions takes on when the zero-order beam is not focused on the information recording layer in the information recording medium, with dashed lines. When the zero-order beam is out of focus on the information recording layer in the information recording medium, the light reception patterns of each of the reflecting zero-order, plus first-order and minus first-order beams are ellipses whose major axes are diagonals of the corresponding light receiving portions A′ to D′, E′ to H′, or I′ to L′. Accordingly, the reflecting zero-order, plus first-order and minus first-order beams are not cast on the light receiving portions A′ to L′ equally. The light reception patters of this case are indicated with dashed lines.
On the basis of outputs from the respective light receiving portions A′ to L′, a focus error signal (hereinafter referred to as an “FE signal”) is produced by performing arithmetic on a mathematical expression of {(an output from the light receiving portion A′+an output from the light receiving portion C′)−(an output from the light receiving portion B′+an output from the light receiving portion D′)}+k[{(an output from the light receiving portion I′+an output from the light receiving portion K′)−(an output from the light receiving portion J′+an output from the light receiving portion L′)}+{(an output from the light receiving portion E′+an output from the light receiving portion G′)−(an output from the light receiving portion F′+an output from the light receiving portion H′)}], where reference sign k denotes a ratio of the light intensity of the zero-order beam to the light intensities of the respective plus and minus first-order beams. On the basis of the thus-obtained FE signal, the focusing control is performed. Thereby, the zero-order beam can be focused on the information recording layer in the optical disc media.
In these years, two-layered optical disc media each having two information recording layers are widely in use as the optical disc media. As shown in FIG. 7B, a two-layered optical medium (a disc 200) has a structure in which a base layer having a first information recording layer 200A formed as a first layer is adhered to a base layer having a second information recording layer 200B formed as a second layer, with an intermediate layer interposed in between. The first information recording layer 200A is formed from a translucent reflection film; reflects parts of the respective total quantities of the zero-order beam, the plus first-order beam, and the minus first-order beam; and transmits the rests of the respective total quantities of the zero-order beam, the plus first-order beam, and the minus first-order beam. The second information recording layer 200B is formed from a reflection film, and reflects the zero-order, plus first-order and minus first-order beams from the first information recording layer 200A. In addition, the focusing control is similarly performed on this multi-layered disc 200 for the purpose of focusing the zero-order beam on each of the information recording layers 200A, 200B. This technology is described, for instance, in Japanese Patent Application Publication No. Hei 4-168631.
In the two-layered optical disc media, it is likely, for example, that when the laser beam (the zero-order beam) is focused on the first information recording layer 200A, the reflecting zero-order beam reflected off the second information recording layer 200B may be cast on the light receiving portions I′, G′, which are expected to receive the reflecting plus first-order beam and the reflecting minus first-order beam, respectively. The hatched area shown in FIG. 7A indicates the area on which the zero-order beam (stray light) reflected off the second information recording layer 200B shown in FIG. 7B is cast.
Once this phenomenon takes place, the outputs from the light receiving portions I′, G′ become greater than their respective expected values, and the focus servo operation using the differential astigmatism method is hindered from being performed appropriately. Particularly, in a case where the light intensity of the zero-order beam is greater than the light intensities of the respective plus and minus first-order beams (in other words, in a case where the value represented by reference sign k is larger), the outputs from the light receiving portions I′, G′ are amplified by a high gain. For this reason, this problem becomes conspicuously serious.
Furthermore, in a case where the pickup apparatus performs a reading operation and the like on a disc complying with the BD standards, a disc complying with the DVD standards and a disc complying with the CD standards, the pickup apparatus needs to include light-receiving elements and laser units for emitting the laser beams to be cast on the different types of discs, respectively. This entails another problem of the increase in the number of parts needed for the optical pickup apparatus.